1. Field of the Invention
This invention relates to regulated, direct current (dc) voltage supplies. More particularly, this invention relates to solid-state, voltage references capable of maintaining dc output voltages each having a high, substantially constant magnitude in the presence of ambient temperature variations.
2. Description of the Prior Art
Some solid-state voltage regulators include internal voltage reference sources and error amplifiers which compare portions of regulated dc output voltages with the output voltages of the reference sources. The reference sources of some regulators rely on the temperature-dependent characteristics of the base-to-emitter voltage (V.sub.BE) of a transistor. For instance, U.S. Pat. No. 3,617,859 of Robert C. Dobkin et al, discloses a reference source which includes a diode-connected transistor operated at one current density and a second transistor operated at a different current density. These two transistors are interconnected with associated circuitry so as to develop a voltage proportional to the difference in their respective base-to-emitter voltages (.DELTA.V.sub.BE). This difference voltage, which has a positive temperature coefficient (TC), is connected in series with the V.sub.BE of a third transistor, which has a negative TC. This resulting structure produced a composite voltage at the output terminal of the reference source. Since the temperature coefficients of the two individual voltages are of opposite sign, the output voltage can be made relatively insensitive to temperature variations by proper choice of circuit parameters. Such circuits have disadvantages relating to the facts that they require at least three precisely matched transistors and produce regulated output voltages equal only to the bandgap voltage of about 1.205 volts or multiples thereof.
U.S. Pat. No. 3,887,863 of Adrian Paul Brokaw, discloses a two transistor voltage reference source wherein the ratio of current densities of the two transistors is automatically controlled to a predetermined value by a nagative feedback amplifier. A voltage having a positive TC corresponding to the .DELTA.V.sub.BE of the two transistors is developed and connected in series with the V.sub.BE voltage of one of the two transistors having a negative TC. The circuit parameters are selected so that the composite output voltage has a low or minimal temperature coefficient. A voltage dividing network comprising two series-connected resistors may be connected to the output terminal of the negative feedback amplifier. The common junction between these resistors provides a reference voltage which is a predetermined fraction of the output voltage. Therefore, the resulting output voltage is a predetermined multiple of the reference voltage.
Although the foregoing circuit of the U.S. Pat. No. 3,887,863 operates satisfactorily for many applications, it does not always operate satisfactorily when required to provide a high level output voltage approximately equal to several bandgap voltages. For example, one industry standard voltage regulator (MC1723) is specified to have an output voltage of 7.15 volts. The multiplication of error signals within simpler versions of the reference circuit of U.S. Pat. No. 3,887,863 provides unacceptable temperature drifts in the regulated output voltage. Other versions of the circuitry of the U.S. Pat. No. 3,887,863 are too complex for some applications.
Other voltage regulators have utilized a zener diode and a transistor junction in series to provide a desired regulated, reference voltage level. If the output voltage is to be centered at a nominal 7.15 volts, the zener diode breakdown voltage must be selected to be 6.47 volts. The positive TC of a 6.47 volt zener commonly used in monolithic integrated circuit structures is about 2.75 millivolts per degree C. but the negative TC of VBE of the transistor is only about 2.2 millivolts per degree C. Thus the composite output voltage has a TC of about 0.55 millivolts per degree C. or 0.008%/.degree.C. This undesirable TC when added to other TC's of the output voltage caused by other circuitry causes the composite TC of the output voltage to be outside of acceptable specifications for the MC1723. Also, a zener diode having a breakdown voltage of 6.47 volts is difficult to process and it exhibits voltage drift with time caused by surface effects.